U.S. patent number 6,896,779 [Application Number 10/857,369] was granted by the patent office on 2005-05-24 for corrosion sensor.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Navy. Invention is credited to Elizabeth A. Hogan, Keith E. Lucas, Paul Slebodnick, Elvin D. Thomas, III.
United States Patent |
6,896,779 |
Thomas, III , et
al. |
May 24, 2005 |
Corrosion sensor
Abstract
A system using tank corrosion sensors to provide for an overall
assessment and monitoring of the electro-chemical corrosion and
coatings condition in ships' tanks, and particularly in ships'
seawater or compensated fuel tanks. The system includes reference
half-cells mounted along a suspended cable and one instrumented
sacrificial anode at the end of the cable to provide optimal
sensing capability within a tank structure. The reference
half-cells and the sacrificial anode measure a potential and
current output, respectively. Together the measurements provide
objective information that can be used to predict corrosion damage
and coating deterioration occurring throughout the structure of the
tank. The system may be used for an overall assessment and
monitoring of the electro-chemical corrosion and coatings
condition. In a preferred embodiment, the measurements are stored
in a datalogger that is optimally contained within an associated
instrument housing. If used with other systems in other tanks, the
system may be used to monitor the relative tank condition, trend
tank condition changes over time, range tank behavior into three
categories and provide a direct analysis methodology for making
tank maintenance decisions.
Inventors: |
Thomas, III; Elvin D. (Ft.
Washington, MD), Lucas; Keith E. (Upper Marlboro, MD),
Slebodnick; Paul (Springfield, VA), Hogan; Elizabeth A.
(Upper Marlboro, MD) |
Assignee: |
The United States of America as
represented by the Secretary of the Navy (Washington,
DC)
|
Family
ID: |
25219804 |
Appl.
No.: |
10/857,369 |
Filed: |
May 21, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
816148 |
Mar 26, 2001 |
6809506 |
|
|
|
Current U.S.
Class: |
204/404;
204/196.02; 204/196.03; 204/196.06; 204/196.07; 204/196.34;
204/196.37; 205/727; 205/728; 205/730; 205/731; 205/740; 205/775.5;
205/776; 205/776.5; 205/777 |
Current CPC
Class: |
G01N
17/02 (20130101) |
Current International
Class: |
G01N
17/00 (20060101); G01N 17/02 (20060101); G01N
027/26 (); C23F 013/00 () |
Field of
Search: |
;204/404,196.02,196.03,196.06,196.07,196.34,196.37
;205/775.5,776,776.5,777,727,728,730,731,740 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bell; Bruce F.
Attorney, Agent or Firm: Karasek; John J. Ferrett; Sally
A.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of Ser. No. 09/816,148, filed
Mar. 26, 2001 now U.S. Pat. No. 6,809,506.
Claims
What is claimed is:
1. A corrosion sensor system comprising: a reference module
designed specifically for use in measuring the electrochemical
potential of the surface of a tank; a measuring module connected to
said reference module for measuring the amount of protection
current necessary for condition based monitoring and long term
corrosion and coatings assessment; and an electronic module
connected to said reference module for monitoring and storing
potential and current data to allow for analysis of tank coatings
degradation.
2. The corrosion sensor system of claim 1, whose reference module
further comprises: a small hermetically sealed non-metallic
enclosure, which contains two cable interconnection connectors, a
metallic threaded stud coupling for mounting and a 5-position
connector for attachment of the reference sensing element.
3. The corrosion sensor system of claim 2, wherein the reference
module includes a multiple plug-in module array having a single
cable interconnection and interconnects variable module quantities
and cable lengths for use in measuring the tank electro-chemical
potential of a surface of a tank at various heights and with
multiple sensor modules, using the two cable interconnection
connectors.
4. The corrosion sensor system of claim 1, whose reference module
determines the fill and empty operational cycle of the tank and
used to document corresponding tank electrolyte depths concurrent
with electro-chemical potential measurements.
5. The corrosion sensor system of claim 1, whose reference module
evaluates the growth of the calcareous deposits on the metal
surface and surface corrosion based on the extent of
polarization.
6. The corrosion sensor system of claim 1, whose measuring module
further comprises: a sacrificial metal which is electrically
isolated from the tank structure using a rigid dielectric barrier
and a connector to enable connection to the single interconnection
cable.
7. The corrosion sensor system of claim 1, whose measuring module
measures, in-situ, an amount of protection current by measuring the
voltage drop across a shunt resistor connected to the tank
structure, thus providing a direct measurement of actual current
necessary to protect the tank.
8. The corrosion sensor system of claim 1, whose measuring module
current measurement are used to define a protection current
requirement, which is supplied by the structure's cathodic
protection galvanic anode system.
9. The corrosion sensor system of claim 1, whose measuring module
data, through direct calculation by Faraday's Law, predicts the
condition of permanently installed cathodic protection anodes and
the anticipated lifetime before exhaustion.
10. The corrosion sensor system of claim 1, whose electronic module
further comprises: an integral datalogger for data storage at the
termination connection of the single interconnect cable.
11. The corrosion sensor system of claim 1, whose electronic module
further comprises a non-metallic enclosure which contains the
datalogger, the location for the shunt resistor and connection for
the hull grounding cable.
12. A corrosion sensor system of claim 1 capable of being utilized
in seawater ballast tanks and compensated fuel tanks and, which can
be utilized to indicate the relative location of coatings damage,
tank protection polarization and prediction of galvanic anode
system life, regardless of fuel properties.
13. A method for analyzing data acquired from the corrosion sensor
system, comprising: storing polarization data from the reference
and measuring modules; storing current data from the reference and
measuring modules; and combining said polarization and current data
from the reference and measuring modules to define a specific range
level indicating relative levels of coating damage and tank
protection.
14. A method according to claim 13, wherein said combining further
comprises: ranking polarization levels of a tank into levels of:
less than -900 mV; between -750 and -900 mV; and greater than -750
mV.
15. The method according to claim 13, wherein said combining
further comprises: ranking current levels of a tank into levels of:
less than 75 mA; between 75 and 175 mA; and greater than 175
mA.
16. A method according to claim 13, further comprising: performing
trend analysis on said data from which, life-cycle maintenance
decisions concerning long-term behavior and changes in the relative
location of coatings damage, tank protection polarization, and
performing prediction of galvanic anode system life to be monitored
and documented in a condition based maintenance approach.
17. A method of installing a corrosion sensor system comprising:
attaching metallic studs to the tank wall at desired module
locations for support; attaching reference modules to the welded
studs of the tank at various heights; attaching measurement modules
near the bottom of the tank; attaching a non-metallic enclosure to
either the underside of the tank access hatch or outside the tank
at an accessible location; and attaching a hull grounding cable to
the tank structure or hull of ship or watercraft.
18. An installation method of the corrosion sensor system reference
module and measuring module, according to claim 17, further
comprising: connecting a desired number of reference modules and
measuring module to the single interconnect cable; connecting an
interconnect cable to the non-metallic enclosure connector;
installing a reference module "sensing element" into one of five
locations provided on the reference cell module connector, as to
indicate specific location with respect to other reference modules
within the tank; and programming the datalogger to incorporate the
number of modules, sampling period and start date.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an corrosion monitoring system,
which is used to provide an overall assessment of the materials
degradation and the condition of protective coatings in a tank
structure in which the metal is subject to corrosion, and
particularly relating to a corrosion sensor for use in tanks which
contain or intermittently contain conductive electrolyte.
2. Description of the Related Art
Shipboard tanks make up a significant percentage of below deck
space in ships and vessels. These tanks are necessary components
for the storage of liquids, for example, ballast seawater,
compensated fuel/seawater, and a number of other essential liquids.
The size and quantity of these tanks vary considerably for each
class of ship. Each tank on a ship has a unique geometry,
operational use and a set of corresponding environmental factors in
which the metals and coatings are exposed. Seawater tanks, used in
many ballasting operations, are subject to high salinity
conditions, high humidity, the attachment of biological materials
to the surfaces and repeated fill/drain cycling. Fuel tanks may be
purely fuel storage or in many cases they are compensated with
seawater, to minimize hull buoyancy changes as the fuel is
consumed. In these compensated tanks, conditions continually vary
between a petroleum-based system to that of seawater immersion.
Other tanks, such as sewage (combined holding tank) and potable
water, are both exposed to unique environments. Tanks are coated
differently depending on usage and may or may not have galvanic
anode cathodic protection, although all tanks with seawater influx
are generally cathodically protected. In addition to basic usage
differences, within each tank corrosion conditions and coatings
performance may vary considerably. In seawater ballast tanks, areas
in residual water are continually immersed in electrolyte and
receive cathodic protection most of the time. The corresponding
vertical wall areas and overheads undergo routine cycling during
normal use and usually have wet/dry conditions along with high
humidity and heat. These tanks also contain a significant
percentage of structural components, which are difficult to prepare
and coat effectively. Overhead coated surfaces, while often wet
from condensation and high humidity, fail by effects of gravity and
osmotic pressure directly at the coatings surfaces. While each of
these areas are exposed to similar conditions, in general, failures
for different surfaces may occur at different rates and by
different mechanisms. Those tanks located on the ship exterior may
additionally receive solar energy and suffer from highly variable
temperature and heat cycling effects.
The maintenance of tanks is more than just re-painting the metal
surfaces. Tank inspection and assessment alone requires the need
for manual opening, gas freeing, staging (if necessary) and entry
of trained personnel. In the U.S. Navy alone, thousands of tanks
are inspected each year, with an average cost of eight to fifteen
thousand dollars per tank. Each tank is typically inspected at
least once every dry dock cycle, or nominally at least every 5 to 7
years depending on service or ship class. Once tanks are identified
for refurbishment, U.S. Navy fleet tank maintenance costs soar to
over $250 million/year. It is most cost effective to perform
maintenance (staging, surface preparation, coatings application,
and galvanic anode replacement) on only those tanks which are in
the worst condition, especially where funds and time are limited.
In order to accurately determine which tanks require maintenance,
all tanks should be monitored, assessed and correctly identified
for maintenance either continually or beforehand, so that the
maintenance that is performed is done only when the condition of
the tank preservation warrants repair.
Typically, a tank preservation system uses dielectric coatings
(e.g. paint) as the primary corrosion barrier and a cathodic
protection system as a secondary measure to minimize coatings
degradation and to prevent galvanic corrosion of the tank
material.
The cathodic protection system for a tank typically consists of a
number of sacrificial anodes, typically made of a strongly
electro-negative metal such as a zinc or aluminum alloy. The
sacrificial anodes are often referred to as "zincs". The
sacrificial anodes are distributed through the tank and
mechanically attached to the tank walls. Adequate cathodic
protection is so beneficial, that in U.S. Navy ships, for example,
the anode type and arrangement are defined by a Navy specification.
By design, these sacrificial anodes are more "electro-negative" or
"anodic" than the tank metal, commonly steel, thus creating a
controlled corrosion cell where the sacrificial anode is consumed
preferentially to the tank structure. Because the sacrificial
anodes are selected to be more negative than most materials, they
will also protect other metal components within the tank (e.g.
piping, valves, cables). The protection afforded the tank metal
also helps minimize premature coatings failure.
The sacrificial anodes are mechanically attached to the tank walls
to prevent them from shifting during ship motions and electrically
grounded to the tank walls to allow for the conduction of current
from the anode to the tank. For good anode performance, anodes are
generally directly mounted to the tank walls/structure. When
immersed, the sacrificial anodes corrode to produce ions in the
electrolyte (fluid in the tank) and correspondingly supplies
electrons (current) through the metallic path to the tank surfaces.
Because the sacrificial anodes supply electrons to the tank
surfaces, a benign chemical reaction occurs at the tank surfaces
using the electrons supplied by the anode, instead of the corrosion
reaction which would occur at the tank walls if the sacrificial
anodes were not present. Ideally, a sufficient number of
sacrificial anodes are distributed throughout a tank, so that all
areas and components within the tank are influenced by the
sacrificial anodes. More sacrificial anodes may be located at the
lower points within a tank with varying fluid levels, such as a
ballast tank, or in areas which need more protection (e.g. near
Cu--Ni piping which passes through the tank or other non-steel
components). Typically, placement of the sacrificial anodes in a
seawater ballast tank cathodic protection system is weighted 2/3
towards the bottom surfaces of the tank.
Even when the tank is protected by a good dielectric coating,
sacrificial anodes play a significant role. No coating system is
perfect, and if a coating is damaged, the exposed bare tank metal
will be subjected to the tank fluid, with the exposed area being
aggressively attacked and corroded. Even if the damage to the
coating is small, corrosion begins, and over time, tends to
undercut the intact coating around the damage thus enlarging the
area of attack and damage. Coatings damage is a progressive event
and a large number of small damage spots can contribute to
significant damage. The installation of cathodic protection helps
to prevent continued damage at bare areas and minimizes the coating
deterioration and undercutting action.
Several events may happen in a tank during the time between tank
maintenance. Over time, the coatings system begins to fail and more
bare area is exposed. Mechanical damage plays a role, but the
coating itself also adsorbs moisture slowly and moisture eventually
reaches the metallic surface where corrosion begins. Imperfect or
poor coating application may accelerate the moisture absorption
effects or target areas which fail sooner. Whatever the failure
mechanism, eventually more and more tank metal area requires
cathodic protection. As demand on the sacrificial anodes increase
to protect more bare area, the sacrificial anodes are consumed
faster, because the sacrificial anodes are required to output
increasingly greater amounts of current. Eventually, tank coatings
failure occurs when the percentage of damage becomes intolerably
high or when the cathodic protection system (sacrificial anodes
within the tank) can no longer supply enough current with which to
protect the amount of bare area.
Maintenance costs in a tank are extremely costly, because the tank
requires staging, grit blasting recoating, and installation of
fresh sacrificial anodes, under controlled environmental conditions
and all in a very difficult non-uniform geometry. Ships with many
tanks cannot repaint all tanks on a routine basis and port
engineers, with highly limited resources, must decide which tanks
must be recoated and when. Tank inspection is necessary in order to
identify whether a tank requires maintenance. Most tank maintenance
problems fall into several categories often related to the
operational aspects of the ship and are roughly identified as: a)
Corrosion/structural damage. b) Osmotic disbondment caused by
condensation on overhead surfaces. c) Coatings degradation caused
by normal deterioration, variable tank levels, wet/dry cycling or
depletion of cathodic protection. d) Failure related to substandard
coatings.
The geometry is often unique for each tank and maintenance
procedures are often complicated by many complex structural members
and baffles. Working conditions within the tanks are often awkward,
difficult, and potentially dangerous.
At present, a "man-in-tank", visual tank assessment must be
performed by a trained tank coatings inspector in order to inspect
the corrosion damage to the tank walls, deterioration of the
coating system, and condition of the sacrificial anodes. This
method of inspection is costly, time-consuming, and typically
subjective in nature. Typically, visual tank inspections require
that each tank be drained prior to inspection, toxic gas-freed
(i.e. per OSHA/NAVOSH requirements) and subsequently certified to
contain an atmosphere suitable for human entry. For each
inspection, an inspector must go into the tank and visually inspect
all tank surfaces and sacrificial anodes. The subjective nature of
a visual inspection and difficulty in observing many areas of the
tanks may result in missed areas, misinterpretation of corrosion
damage, or poor assessment of general coatings deterioration.
With the economic trend toward increased time between overhauls and
decreased maintenance costs, it is particularly important that tank
conditions be monitored carefully, so that tanks with the greatest
maintenance requirements are correctly identified. Optimally, an
inspection scenario would rate all the tanks, examine the coatings
degradation "trends" within the group and target those tanks within
the population that are in the worst condition. Ideally, to perform
this task and defray the manned inspection costs, a tank corrosion
monitoring system would be available to reduce or eliminate the
costly and time consuming visual inspections. The tank corrosion
monitoring system could be part of a condition based maintenance
plan that would monitor the coatings degradations, analyze data
from tank sensors, and compare and trend the tank conditions
relative to each other. Further, such a fast, inexpensive tank
monitoring and inspection system would allow scarce resources to be
devoted to actual tank maintenance, rather than to labor intensive
visual inspection.
Because opening and preparing a tank for human entry is so
expensive and time consuming, it is optimal to minimize manned
inspections and best to schedule all tank repair and coating work
possible within the period the tank is staged and available.
Typically ship maintenance is planned months prior to arrival of
the ship, requiring schedulers to either estimate tank maintenance
needs based on historic tank data, or on tank inspection reports,
if they are available. If tank maintenance is incorrectly
scheduled, based upon inaccurate and dated human inspections,
unnecessary funds may be expended to refurbish areas that do not
have critical need, and other necessary-maintenance, which had been
deferred in favor of the tank maintenance, may go undone.
Two major sources of data are available to the corrosion engineer
concerning the condition of the tank coatings and the cathodic
protection, without the need for extensive instrumentation. First,
the electrochemical potential of protected steel can be measured
using a standard half-cell, such as a silver/silver chloride
(Ag/AgCl) reference cell, as discussed by H. H. Uhlig, "Corrosion
Handbook" (1955), the disclosure of which is incorporated by
reference. Where steel is protected by a zinc galvanic anode
system, any bare steel surfaces and even the coated steel surfaces
are polarized in an electro-negative direction forcing the steel
surfaces to become cathodic, with respect to the galvanic anode. As
long as sufficient anode mass is correctly located within the
structure and the cathodic area requiring protection does not
exceed the current capacity of the sacrificial anodes, then the
surfaces will remain protected, as discussed in J. Morgan,
"Corrosion Protection", 1960, the disclosure of which is
incorporated by reference. Changes in either of these states can be
measured using appropriate reference half-cells installed in the
tank. No convenient, long term monitoring system is available using
standard half-cells, however.
Second, each galvanic (sacrificial) anode supplies electrical
current as its part in protecting the metal (typically steel)
structure. Measuring this level of electrical current allows a
determination of how active the sacrificial anodes are, and the
level of current and can be used with Faraday's law to predict
anode weight loss and thus predict anode life, based on the rate of
anode deterioration. A special purpose instrumented anode can be
designed whereby the current output can be measured and
subsequently gauged depending on the cathodic protection
requirements of the tank. This special purpose sacrificial anode
does not need to replace an existing sacrificial anode within the
tank, but may be added to the tank in order to measure the
necessary data.
A tank corrosion monitoring system that accurately monitored the
coatings degradations and corrosion level which measures the
current output from an instrumented sacrificial anode and measures
the potential from at least one reference half cell is disclosed
herein.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a
tank corrosion sensor system in which a monitoring and overall
assessment of the electrochemical corrosion and coatings condition
in a liquid storage tank is provided.
Objects of the present invention are achieved by providing an
apparatus which include a half-cells measuring a potential of a
tank. The measured potential indicates an amount of corrosion of
the tank and the level of tank protection provided by the coatings
and cathodic protection system.
Objects of the present invention are achieved by providing an
apparatus which includes an anode measuring a current output of a
tank. The measured current output indicates an amount of corrosion
of the tank and the amount of tank coating degradation.
Objects of the present invention are achieved by providing an
apparatus which includes half cells measuring a potential which
corresponds to a polarization of a tank. The apparatus also
includes an anode measuring a current output of the tank. The
polarization and the measured current output together indicates an
amount of corrosion of the tank and a level of tank protection
provided by the coatings and cathodic protection system.
Objects of the present invention are achieved by providing a method
which includes measuring a potential which corresponds to a
polarization of a tank. The method also includes measuring a
current output of the tank. The polarization and the measured
current output together indicates an amount of corrosion of the
tank and the amount of tank coatings loss.
Objects of the present invention are achieved by providing an
apparatus which includes first means for measuring a potential
which corresponds to a polarization of a tank. The apparatus also
includes a second means for measuring a current output the tank.
The polarization and the measured current output together indicates
an amount of corrosion to the tank and the amount of tank coatings
loss.
Another object is to provide a fast, objective, effective method
for easily comparing ship tanks according to which is most in need
of maintenance.
Another object is to provide a corrosion monitoring system which is
easily integrated into a condition based monitoring program for a
ship.
Another object is to provide a method for evaluating the condition
of ship tank coatings so tanks requiring maintenance are
objectively identified and ranked in order of greatest need.
Additional objects and advantages of the invention will be set
forth in part in the description which follows, and, in part, will
be obvious from the description, or may be learned by practice of
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the invention will become
apparent and more readily appreciated from the following
description of the preferred embodiments, taken in conjunction with
the accompanying drawing of which:
FIG. 1 is a diagram illustrating a tank corrosion monitoring system
according to an embodiment of the present invention.
FIG. 2 is a diagram illustrating an instrumented sacrificial anode,
according to an embodiment of the present invention.
FIG. 3 is a diagram illustrating potential (E.sub.corr) in negative
volts (tank potential referenced to the potential of Ag/AgCl half
cell) plotted against the cathodic surface area of a tank.
FIG. 4A is a diagram illustrating a tank polarization analysis for
a tank in good condition, according to an embodiment of the present
invention.
FIG. 4B is a diagram illustrating instrumented sacrificial anode
current output analysis for a tank in good condition, according to
an embodiment of the present invention.
FIG. 5A is a diagram illustrating a tank polarization analysis for
a tank beginning to deteriorate, according to an embodiment of the
present invention.
FIG. 5B is a diagram illustrating a instrumented sacrificial anode
current output analysis for a tank beginning to deteriorate,
according to an embodiment of the present invention.
FIG. 6A is a diagram illustrating a tank polarization analysis for
a tank in an advanced state of degradation, according to an
embodiment of the present invention.
FIG. 6B is a diagram illustrating an instrumented sacrificial anode
current output analysis for a tank in an advanced state of
degradation, according to an embodiment of the present
invention.
FIG. 7 is a diagram illustrating tank polarization test results for
several tanks, according to an embodiment of the present
invention.
FIG. 8 is a diagram illustrating current output test results for
several tanks, according to an embodiment of the present
invention.
FIG. 9 is a graph of tank polarization test results for tank
filling episode in a tank with 9 to 10 year old tank protective
coating.
FIG. 10 is a graph of tank polarization test results for tank
filling episode in a tank with 1 to 2 year old tank protective
coating.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference will now be made in detail to the present preferred
embodiments of the present invention, examples of which are
illustrated in the accompanying drawings, wherein like reference
numerals refer to like elements throughout.
FIG. 1 is a diagram illustrating a preferred embodiment of a tank
corrosion monitoring system 1 for use within a tank 10, according
to a preferred embodiment of the present invention. The corrosion
monitoring system 1 is a self contained package intended for
in-situ installation within an individual ballast or compensated
fuel tank. The corrosion monitoring system 1 includes: two
reference half cells 2a and 2b, an instrumented sacrificial anode
3, a cable 4 for suspending the reference half cells 2a and 2b
within the tank 10, a magnetic cable tensioner 5, a datalogger 6
for storage of voltage and current data, and a waterproof
electronics enclosure 18.
The two potential reference half-cells 2a and 2b, shown in FIG. 1,
are Silver/Silver Chloride (Ag/AgCl) seawater reference half-cells
(sensors). The half-cells are placed at different levels of the
tank, in order to gather data at different tank levels. A half-cell
2a measures the potential E.sub.corr of the tank at the location
near the half-cell 2a.
The potential across each reference half-cell 2a and 2b is carried
in a wire, which is optimally within cable 4. Cable 4 is suspended
vertically in the tank 10 and is magnetically attached to the
bottom of the tank by a magnetic tensioner 5 to reduce cable 4
movement. The tank magnetic tensioner 5 is a 130 lb pull ceramic
magnet, although other attachment means may be used. The length of
the cable 4 is selected to correspond with the geometry and size of
the tank 10. Optimally, the cable 4 is suited to its environment,
being, for example, resistant to corrosion and wear and meeting
requirements for fuel tank service or seawater SWU (smoke,
waterproof, underwater) specification requirements. The cable 4
includes sufficient wires for carrying electric current from the
instrumented sacrificial anode 3 and the potential across each
reference half-cell 2a and 2b. In this embodiment, the cable 4 was
a four-wire cable, although three wires would have been
sufficient.
The instrumented sacrificial anode 3 is also attached to an end of
the cable 4. The length of the cable, therefore, takes into
consideration the desired location of the instrumented sacrificial
anode 3, the distance required for a strain relief loop 8, and the
ease of removing a tank hatch 7 to access the datalogger 6.
The datalogger 6, is contained in a waterproof electronics
enclosure 18, which is typically mounted on the inside surface of
the tank hatch 7. The datalogger 6 records potential measurements
of the reference half-cells 2a and 2b and current output of the
sacrificial anode 3.
Optionally, the electronics enclosure 18 can include additional
instrumentation, such as a tank level indicator (TLI) datalogger
(not shown). Alternatively, a separate tank level indicator
datalogger may be contained in a separate electronics
enclosure.
The reference half-cells 2a and 2b are suspended within tank 10
with the lower reference half-cell 2b residing near the tank bottom
and the upper reference half-cell 2a arranged near the middle of
the tank to correspond to intermediate and filled states of the
tank. Upon filling the tank with seawater, for example, the lower
reference half-cell 2b registers a change in a potential almost
immediately as the tank 10 fills. Accordingly, the upper reference
half-cell 2a begins to read a potential once water reaches it.
During the fill episode, the sacrificial anodes within the tank
(the tank cathodic protection system) have increasingly more wet
tank area to protect and thus respond by providing more current.
The effectiveness of the sacrificial anodes in protecting the tank
from the electrolyte, as the tank fills and stabilizes, may be
estimated by the potential across the reference half-cells 2a and
2b. Increasing the number of reference half-cells will provide more
refined data concerning the anode cathodic protection performance
and tank condition, although two reference half-cells s supply a
significant amount of information. Analysis of the differential
potential measured between the reference half-cells 2a and 2b, for
example, may provide information about the direction of current
flow, the potential distribution within the tank, the general
location of surfaces requiring the greatest current demand and,
therefore, indirectly, the location of the most significant
coatings deterioration.
The placement alone of two half-cells at different heights within
the tank would provide tank fill data, as the reference half-cell
reads a potential when it contacts the seawater electrolyte. In
compensated fuel tanks, the reference half-cells additionally can
distinguish between fuel and seawater. Note that although only two
reference half-cells are shown in FIG. 1, in other embodiments,
more reference half-cells may be used. Between one and six
reference half-cells are believed to be sufficient for most Navy
ship tanks.
FIG. 2 is a diagram illustrating an instrumented sacrificial anode,
according to an embodiment of the present invention.
In the embodiment shown in FIG. 2, the instrumented sacrificial
anode 3 is isolated from the tank 10 metal by a 1/2" thick PVC
plate 13 with length and width dimensions greater than the
instrumented sacrificial anode 3 dimensions. The instrumented
sacrificial anode 3 is attached to the tank structure 10 by two 110
lb mounting magnets 17, securing bolts 31 and 32. Electrical
connection 19 is for electrical attachment between the anode wire
34 and the anode 3. Typically, anode wire 34 is integrated within
the cable 4. Note that the 1/2" PVC plate 13 could have been
replaced with some other non-metallic material to electrically
isolate the instrumented sacrificial anode 3 from the tank 10.
In order to provide a low resistance ground connection, the anode
wire 34 is attached to the sacrificial anode 3 at electrical
connection 19. The anode wire 34 is of sufficient gauge to carry
the magnitude of current without a voltage drop, typically
equivalent to that normally provided by the anode at a direct
ground metallic connection. The sacrificial anode wire 34, here
contained within cable 4 (shown in FIG. 1), connects through the
reference half-cell and connects directly to a shunt resistor 9.
The shunt resistor of this embodiment is a low wattage (1-3 Watts),
very low resistance (0.1 ohm) resistor and does very little to
impede the flow and magnitude of current to ground. Because the
shunt resistance is low, the slight voltage drop read across the
shunt resistor 9 can be equated directly to the instrumented
sacrificial anode current. Electrical leads 33 attached to the ends
of the shunt resistor feed into the datalogger 6 and provide both a
hull ground reference point and anode current output data, which
are stored by the datalogger.
Anode wire 34 within cable 4 enters the waterproof container 18 via
a penetration in the watertight bulkhead 23 and correspondingly
exits after the shunt resistor 9 in the same manner.
Typically, the instrumented sacrificial anode 3 is selected so that
it will behave nearly identically to the actual tank sacrificial
anodes, which are distributed in various areas of the tank 10. The
instrumented sacrificial anode 3 shown in FIG. 2 is a type ZHC-24
zinc anode, manufactured in accordance with military specification
MIL-A-18001J (a commonly used reference specification for
sacrificial anodes).
The current output measurement obtained from the instrumented
sacrificial anode 3 provides information on the electrical current
required to cathodically protect the nearby tank 10 structure. The
cathodic current demand of the tank metal, to which both the
instrumented sacrificial anode 3 and the tank's sacrificial anodes
respond, can be directly correlated to the condition of the tank
protective coating system, because poor coatings or high bare area
percentages will require more sacrificial anode current to protect.
The instrumented sacrificial anode 3 current output may be
monitored over time to identify relative changes in the integrity
of the tank coatings. During fill episodes of the tank with
seawater, the instrumented sacrificial anode 3 responds to the
increased surface area under immersion. Typically a tank will
require a high current demand immediately after filling until the
surfaces equilibrate and establish a stable film. Once stable, the
current from the tank's sacrificial anodes drops to what is called
a "maintenance current density", which is generally much lower in
magnitude and relatively unchanging. Conversely, sacrificial anodes
that are unable to sufficiently polarize the structure because of
excessive coatings damage, will work at maximum output with very
little current drop-off until they are depleted. Information about
the current output of a tank's sacrificial anodes can be utilized
to aid in assessing coatings damage percentages, damage location,
tank condition change over time, anode life prediction and overall
anticipated coatings life prediction.
The datalogger 6 typically has multiple channels of analog voltage
signal recording and can convert information to digital format for
display and plotting. Sufficient analog to digital (A/D) channels
are typically included to support the potential measurement from
the reference half-cells 2a and 2b and the current output
measurement from the instrumented sacrificial anode 3. The DC
voltage channels within the datalogger 6 that are used for
potential recording typically have minimum resolutions of 0.2 m
VDC, and the channels used for instrumented sacrificial anode
current output recording typically have minimum resolutions of 0.1
m VDC. Most dataloggers 6 may be set to record at intervals from
between 15 times a second to once per day. Typically, however, a
datalogger 6 is set to one data reading per hour for each sensor.
The datalogger embodiment shown in FIGS. 1 and 2 is battery
powered, and preferably has at least 1.5 years of dynamic data
storage capacity consistent with the one reading per hour data
rate. The unit has a data downloading capability to accomodate easy
data retrieval from the hatch or other installed location.
When an optional tank level indicator is used, preferably it will
be programmed to collect data at a similar interval (e.g. once
every hour), so it may easily be correlated with the current output
and potential data.
Optionally, the electronics enclosure may contain only a wire
junction box, without a datalogger 6, when the system 1 is
electrically wired directly to a ship data storage system outside
the tank 10. Alternatively, the wires carrying the voltage and
current from the half-cells 2a and 2b and the instrumented
sacrificial anode 3 may be routed directly through bulkhead
penetrations to an electronics enclosure 18 and datalogger 6
located outside the tank.
Once the system 1 is installed and set to operate, the tank hatch 7
is closed and the tank 10 is sealed for normal operation. To
collect data from a hatch mounted 7 configuration, as shown in
FIGS. 1 and 2, the hatch 7 is opened and the datalogger 6 accessed
by opening the sealed electronics enclosure 18. No manned entry
into the tank is required to read a datalogger 6, as the hatch 7
typically can be removed and placed on the deck outside of the
tank. In a preferred embodiment, the data is collected from the
datalogger 6 via an RS232 serial connection on the electronics
enclosure 18.
Once collected, the data may be reduced in standard spreadsheet
format and graphed for analysis. The following data are typically
collected: (1) time to polarization, (2) current output of the
instrumented sacrificial anode, (3) polarization level of the tank,
(4) number and levels of tank fill episodes, and (5) reference
half-cell differential.
The measurement of electro-chemical potential provides a
significant amount of information concerning the state of overall
tank preservation. In FIG. 3, the tank potential (E.sub.corr)
referenced to a Ag/AgCl half-cell is plotted against the cathodic
surface area for a steel tank having 1.2 sq ft of sacrificial zinc
anodes for cathodic protection. The cathodic surface area is that
area of the tank 10 where coatings have deteriorated or where tank
metal is exposed to the liquid in the tank. FIG. 3 illustrates how
increased cathodic surface area affects the protection potential of
the tank. In real terms, the tank contains a finite amount of
sacrificial anodes and as the coatings deteriorate the cathodic
surface area increases, as indicated. A rise in cathodic surface
area results in the decrease in protection levels for a typical
sacrificial anode system. More precisely, a tank with little
coatings damage would have potentials near -1.0 V, while one with a
large coatings damage percentage would have potentials nearer to a
freely corroding steel potential of -0.7 V. For a given
distribution of sacrificial anodes in a tank, such as the 1.2
square foot, illustrated in FIG. 3, the sacrificial anodes have
only a finite amount of current capacity available to protect the
coated tank surfaces. As the cathodic area increases, (i.e. a
deterioration in coated area) the overall potential of the tank
begins to fall off toward more electro-positive potentials. At
significant coatings damage percentages, the cathodic protection
system (the array of sacrificial anodes) is no longer able to
maintain potentials at sufficiently negative levels to effectively
protect the tank surfaces, and from that point, coatings
deterioration will progress at an accelerated rate. Potential
measurements, thus, provide a good indication of tank condition,
regardless of the method of coatings failure, because the cathodic
protection system will compensate for coatings changes.
If a tank has been recently refurbished (i.e. painted with a good
dielectric coating), it will have very little surface area to
protect and thus reference half-cells will display potentials at or
near the reference levels of the sacrificial anodes. As coatings
deteriorate, the rate of polarization during filling of a tank will
remain fairly rapid except in two cases. First, there may be such a
high percentage of tank coating damage that the sacrificial anodes
are no longer able to polarize the structure. Hence, the reference
half-cell potentials would begin to drift more electropositive, as
indicated in FIG. 3. Second, the sacrificial anodes will gradually
be depleted over time to the point that the remaining anode mass
has insufficient current capacity to polarize the structure. The
use of two or more reference half-cells in the tank, however,
provides the ability to track trends in the potential behavior and
to compare variations between individual half-cells 2a and 2b. An
analysis of differential reference half-cell readings can provide
some indication as to coatings damage location, especially where
multiple readings or a definite trend has been identified. If
damage is uniform throughout the tank, then the reference
half-cells will likely read similar potentials and correspondingly
have similar rates of polarization. As the damage becomes more
localized, the half-cell nearest the failed coatings area will
typically shift more electro-positive than the remaining
half-cells, thus identifying coatings disparities within the
tank.
FIGS. 4-6 will illustrate the use of potential measurement and
instrumented sacrificial anode current output to determine the
condition of tank coatings and sufficiency of the cathodic
protection system. The figures show schematic representations of
how tank properties change when a tank is filled with a liquid.
FIGS. 4A, 5A, and 6A show a typical polarization scenario of the
tank (as measured by a silver/silver chloride half-cell according
to the invention) plotted against time, as the tank is filled and
remains full. The resultant polarization provides not only the
extent of polarization (level of cathodic protection), but also
identifies those tanks that polarize immediately verses those which
polarize slowly. Given the fixed tank area and an initial state,
each filling episode provides a new polarization curve
representative of conditions that currently exist and
correspondingly provides trend data for long-term prediction. FIGS.
4B, 5B, and 6B show the current output as measured from an
instrumented sacrificial anode, corresponding to FIGS. 4A, 5A, and
6A, respectively. At a filling event, the current demand is
initially higher and subsequently drops as the surfaces become
polarized and less current is required.
FIG. 4A is a diagram illustrating a tank polarization analysis for
a newly refurbished tank being filled with a liquid (typically
seawater), according to an embodiment of the present invention.
Referring now to FIG. 4A, as the tank is filled, the silver/silver
chloride potential sensor begins to read when it becomes immersed
in seawater, near time zero. Curve 42 portrays the rapid
polarization of the tank, from levels near freely corroding steel
(-0.6 V), in a negative direction, to values approaching -1.0 V,
which is near the maximum zinc anode potential. Potential values
more negative than about -0.9 V indicate that minimal or no coating
deterioration has occurred, that very little corrosion damage can
proceed, and that the tank requires no maintenance.
FIG. 4B is a diagram illustrating the corresponding instrumented
sacrificial anode current output data curve 44 for the same
recently refurbished tank. Because the tank has been recently
refurbished, the current output of the instrumented sacrificial
anode is low, since only minimal current is required to polarize
the structure. When the tank is filled, the current required by
this anode spikes initially, but only to a value less than about
1/3 third of the maximum anode capacity. Immediately, as the tank
polarizes, the current begins to drop-off and stabilizes at
approximately 75 mA, this stable level referred to as the
"maintenance current density". Three factors are of primary
importance in an analysis of the curve: the magnitude of maximum
current output, the drop off rate, and the maintenance current
density level. Each of these values contributes information
concerning tank coatings damage percentages, the ability of the
cathodic protection system to protect the structure, and projected
anode life. Examination of the current output of FIG. 4B and
potential measurement of FIG. 4A provide more information than
either FIG. 4B or FIG. 4A alone.
FIG. 5A is a diagram illustrating a tank polarization analysis for
a tank with a moderate amount of corrosion/coatings damage being
filled with liquid (seawater). Referring now to FIG. 5A, the curve
52 is representative of the same layout as that discussed
previously. Because the tank has moderate levels of coatings
damage, there is a greater percentage of uncoated steel which
requires protection. It would, thus, be anticipated that the
sacrificial anodes would be required to supply more current, than
seen in FIG. 4B, in order to polarize the structure. FIG. 5A
reflects this difference in tank condition, because the time to
polarization is increased and the level achieved is only
approximately -0.8 V. This level of polarization indicates that the
tank is adequately cathodically protected, however, it is likely
that further coatings deterioration will lead to less protection
and subsequently, to greater sacrificial anode material loss. FIG.
5B is a diagram illustrating instrumented sacrificial anode current
output analysis for the same steel tank with a moderate amount of
corrosion/coatings damage being filled with liquid (seawater).
Correspondingly, curve 54 of FIG. 5B shows that the initial anode
current required to polarize the structure is high--near the
maximum anode output level of -400 mA. In addition, the current
drop-off is slower to occur. It can be observed that the
"maintenance current density" value of approximately 175 mA is at a
greater value than that shown in FIG. 4B, indicating that the
cathodic protection system must work harder to protect the tank,
and allowing the conclusion that the tank must have some moderate
level of coatings damage. It is likely that the remaining
sacrificial anodes in the tank are currently adequate to protect
the tank. It may be inferred that the sacrificial anodes will be
depleted at a faster rate, and that they will require replacement
nearer in the future. A reliable quantitative prediction of anode
life may be calculated from the current and using Faraday's
law.
FIG. 6A is a diagram illustrating a tank polarization analysis for
a severely corroded tank being filled with seawater. Referring now
to FIG. 6A, the steel tank 10 is in a condition where the cathodic
protection system is unable to polarize the structure because there
is an excessive amount of coatings damage. The curve 62 does not
approach the -1.0 V level, and in fact, shows almost no tank
polarization, thus indicating that the steel remains at a freely
corroding potential where severe corrosion and continued rapid
coatings deterioration is likely. The potential measurement is well
below a specific level desired for even minimal cathodic
protection. FIG. 6B is a diagram illustrating a instrumented
sacrificial anode current output analysis for the same severely
corroded tank being filled with seawater, according to an
embodiment of the present invention. The instrumented sacrificial
anode curve 64 confirms the fact that the tank coatings are in a
severely damaged state and that the steel cannot be polarized by
the present cathodic protection system. The initial current output,
as shown in the first portion of curve 64, rapidly reaches the
anode maximum output level of approximately 400 mA and drops off
only minimally to approximately 375 mA. This drop-off level is not
a "maintenance current density", as evidenced from the inability of
the sacrificial anodes to polarize the tank seen in curve 62. It
would be presumed that the remaining anode material would be
depleted rapidly. Again, a reliable quantitative prediction of
anode life may be calculated from the current using Faraday's
law.
Another factor that enters into long range prediction is the fact
that as a coating ages, the dielectric properties begin to
gradually breakdown and even though the coating has not visually or
physically failed, the reduced barrier properties also place
increasing demand on the cathodic protection system to protect
large coated surfaces of the tank 10. As with a coatings failure to
bare metal, the current output of the sacrificial anodes ultimately
increases until a maximum output level is obtained and the cathodic
protection system can no longer maintain the same level of
polarization within the tank. This condition, very similar to that
shown in FIGS. 6A and 6B, would indicate that the coating system
retains little if any barrier capability, that the tank is no
longer protected by the coating, and that coating replacement is
required immediately.
FIG. 7 illustrates how condition ranking of tanks may be
accomplished, and is a diagram illustrating actual test results
(tank potential measurements over a period of time) from various
test installations on different ship tanks. FIG. 7 shows potential
data obtained from the upper reference half-cell acquired from five
different ship tanks, using the two reference half-cell
configuration. The five curves were taken during a single filling
event and clearly discerned different tank states. The potential
levels were graded into three condition rankings, which
corresponded to a traffic light scenario. "Green" tanks were
considered to be trouble free (more electro-negative than about
-900 mV) and required no maintenance. Tanks which fell into a
"yellow" zone (about -750 mV to about -900 mV) were indicative of
increased activity placed on the cathodic protection system and had
the requirement for additional current to protect more bare or
degrading coatings area. Tanks with nearly freely corroding
conditions, fell into the "red" zone (more electro-positive than
about -750 mV) and had an unacceptable percentage of corrosion
damage. Additionally, the "red" tanks most likely had a failed or
significantly overworked cathodic protection system.
FIG. 8 is a diagram illustrating actual prototype instrumented
sacrificial anode results from test installations aboard various
different ship tanks. The "condition ranking" scenario is an aspect
of the embodiment of the invention. In FIG. 8, the output current
from an instrumented sacrificial anode is plotted verses time in
hours and corresponds with potential data shown in FIG. 7. The
tanks with newly painted surfaces and low cathodic protection
requirements drew a minimal amount of current from the sacrificial
anodes. Values for the initial current demand and subsequent
drop-off associated with calcareous deposition (stable surface
films), were measured and utilized to provide an indicator for
long-term requirements on the system. In the tanks where some
coatings breakdown had occurred, the sacrificial anodes responded,
as expected, and provided an increasing level of current. Once the
zinc "maintenance current" output exceeded 75 mA, that tank
condition was degraded to the yellow condition state and
correspondingly, when the output exceeded 175 mA the condition was
changed to a red state.
The curves in FIGS. 9 and 10 show an example of a data set for a
filling episode in two tanks with widely variable coatings
conditions. FIG. 9 shows potential test data 92 taken from
reference cells and current data 94 reported from the instrumented
sacrificial anode in a tank with a moderate level ("yellow"
condition) of damage. FIG. 10, shows test data plotted as potential
curve 102 and current curve 104 from an adjacent tank on the same
ship, with similar geometry and the same quantity of zinc
sacrificial anodes, except that this tank had recently been
refurbished and had both a good coatings system and good cathodic
protection. The instrumented sacrificial anode and reference
half-cells were installed in relatively the same locations in both
tanks, with the reference half-cells located 1 m above the bottom
and 3 m above, respectively.
FIG. 9 represents data for a 9-10 year old tank coating, while FIG.
10 shows data from a 1-2 year old coating system. In the
deteriorating tank condition shown in FIG. 9, the curve 92
indicates that the tank polarized very slowly and did not reach the
same level of polarization nor a steady state level of
polarization. The corresponding zinc current curve 94 showed an
initial spike nearly 4 times that of the newer system of FIG. 1,
followed by a gradual decline in current output that mirrored the
slow polarization progress of curve 92. The final maintenance
current output, of approximately 150 mA, was still 3 times that of
the newly coated tank for the same duration, indicating a high
current demand, and a moderate level of tank coatings damage.
In FIG. 10, the polarization curve 102 (from the reference
half-cells) showed immediate tank polarization along with a
corresponding initial spike in the current from the instrumented
sacrificial anode 104. With only minimal current necessary to
polarize the tank, the current demand curve dropped to a low steady
maintenance current of approximately 50 mA, indicating almost no
damage to the tank coatings.
In a preferred embodiment of the invention, a reference half-cell
is part of a "plug-in" sensor module. The sensor module includes a
reference half-cell and connection points which are easily
connected to a length of cable. These sensor modules make
installation of the system with various numbers of reference
half-cells into a tank much easier and faster, and allow quick
changeout of reference half cells when necessary.
In an embodiment of the invention, the tank corrosion monitoring
system is used in a condition based maintenance method which
monitors tank corrosion and coating condition for a number of
tanks, ranks the condition of the tanks, and predicts trends. The
data provided by the tank monitoring system is used to determine,
for example, the status of coatings and cathodic protection
systems, the basic location of the coatings damage, the ability of
the cathodic protection system to protect the tank, the predicted
remaining life of the sacrificial anodes, and the percentage of
coatings damage. Data from different tanks is compared and each
tank is ranked according to its relative damage and condition.
These trend data are used to determine the tank maintenance needs
of each ship, without the need for manned entry or periodic visual
inspections. This method works with either good-moderate-poor
analysis of the tank conditions or with a detailed analysis of each
tank. Results are objective in nature and fully documentable. As
part of an overall ship husbandry system, this method can
significantly lower costs and shorten ship maintenance times.
In another embodiment, instrumented sacrificial anodes and
reference half-cells are installed as a part of an integrated ship
tank monitoring system. These components also may be integrated
into computer systems which monitor the condition of the ship.
One embodiment of the invention is directed to a corrosion sensor
system that includes a reference module designed specifically for
use in measuring the electrochemical potential of the surface of a
tank, a measuring module connected to the reference module for
measuring the amount of protection current necessary for condition
based monitoring and long term corrosion and coatings assessment,
and an electronic module connected to the reference module for
monitoring and storing potential and current data to allow for
analysis of tank coatings degradation.
In an embodiment, the reference module can include a small
hermetically sealed non-metallic enclosure, which contains two
cable interconnection connectors, a metallic threaded stud coupling
for mounting and a 5-position connector for attachment of the
reference sensing element. The reference module can also include a
multiple plug-in module array having a single cable interconnection
and interconnects variable module quantities and cable lengths for
use in measuring the tank electro-chemical potential of a surface
of a tank at various heights and with multiple sensor modules,
using the two cable interconnection connectors.
The reference module can determine the fill and empty operational
cycle of the tank and used to document corresponding tank
electrolyte depths concurrent with electro-chemical potential
measurements. Further, the reference module can evaluate the growth
of the calcareous deposits on the metal surface and surface
corrosion based on the extent of polarization.
The measuring module can include a sacrificial metal that is
electrically isolated from the tank structure using a rigid
dielectric barrier and a connector to enable connection to the
single interconnection cable. The measuring module can measure,
in-situ, an amount of protection current by measuring the voltage
drop across a shunt resistor connected to the tank structure, thus
providing a direct measurement of actual current necessary to
protect the tank. Moreover, the measuring module current
measurements can be used to define a protection current
requirement, which is supplied by the structure's cathodic
protection galvanic anode system. The measuring module data,
through direct calculation by Faraday's Law, can predict the
condition of permanently installed cathodic protection anodes and
the anticipated lifetime before exhaustion.
The electronic module can also include an integral datalogger for
data storage at the termination connection of the single
interconnect cable. The electronic module can also include a
non-metallic enclosure which contains the datalogger, the location
for the shunt resistor and connection for the hull grounding
cable.
The corrosion sensor system can be utilized in seawater ballast
tanks and compensated fuel tanks and, can be utilized to indicate
the relative location of coatings damage, tank protection
polarization and prediction of galvanic anode system life,
regardless of fuel properties.
Another embodiment of the invention is directed to a method for
analyzing data acquired from the corrosion sensor system, including
storing polarization data from the reference and measuring modules,
storing current data from the reference and measuring modules, and
combining the polarization and current data from the reference and
measuring modules to define a specific range level indicating
relative levels of coating damage and tank protection. Combining
the polarization and current data can include ranking polarization
levels of a tank into levels of (a) less than -900 mV; (b) between
-750 and -900 mV; and (c) greater than -750 mV. The current levels
in a tank can be ranked into levels of: (a) less than 75 mA; (b)
between 75 and 175 mA; and (c) greater than 175 mA.
The method can also include performing trend analysis on said data
from which, life-cycle maintenance decisions concerning long-term
behavior and changes in the relative location of coatings damage,
tank protection polarization, and performing prediction of galvanic
anode system life to be monitored and documented in a condition
based maintenance approach.
Another embodiment of the invention is directed to a method of
installing a corrosion sensor system including attaching metallic
studs to the tank wall at desired module locations for support,
attaching reference modules to the welded studs of the tank at
various heights, attaching measurement modules near the bottom of
the tank, attaching a non-metallic enclosure to either the
underside of the tank access hatch or outside the tank at an
accessible location, and attaching a hull grounding cable to the
tank structure or hull of ship or watercraft.
The method of installing the corrosion sensor can also include
connecting a desired number of reference modules and measuring
module to the single interconnect cable, connecting an interconnect
cable to the non-metallic enclosure connector, installing a
reference module sensing element into one of five locations
provided on the reference cell module connector, as to indicate
specific location with respect to other reference modules within
the tank, and programming the datalogger to incorporate the number
of modules, sampling period and start date.
Although the examples provided herein primarily identify tanks as
being tanks within a ship, the invention is not so limited. The
systems and methods described herein are equally applicable to
other tanks which contain or intermittently contain conductive
electrolyte, on other types of vessels, or in stationary
applications.
Various numerical values and ranges are described herein, however,
the present invention is not limited to such values and ranges.
Instead, it should be understood that such values and ranges are
only examples of specific embodiments of the invention.
Although a few preferred embodiments of the present invention have
been shown and described, it would be appreciated by those skilled
in the art that changes may be made in these embodiments without
departing from the principles and spirit of the invention, the
scope of which is defined in the claims and their equivalents.
* * * * *